Sound recording system



Feb. 11, 1936. I. M, CHAMBERS SOUND RECORDING SYSTEM Filed Sept. 15;1952 INVENTOR M. CHA MBERS FIGS Patented Feb. 1 1, 1 936 UNE'EE @TATESATEN'I ore'is SOUND RECORDING SYSTEM Application September 15, 1932,Serial No. 633,263

10 Claims.

This invention relates to recording systems and particularly to systemsfor recording modulated currents on a photographic medium.

The object of the invention is a system for recording modulated currentsin which a greater range of amplitudes of the modulated currents may berecorded than may be recorded by known systems.

A feature of the invention resides in the method of separately recordingthe half-waves of the variations in amplitude of the modulated currentsto form separate records. As each halfwave may be permitted to vary overthe whole available part of the characteristic of the recording medium,a greater range of amplitudes may be recorded.

Another feature of the invention resides in the method of recording eachhalf-wave, so that when the amplitude of a half-wave is very small, themean exposure of the photographic medium is at the lower end of theregion of correct exposure for the photographic medium. For waves ofsmall amplitude, a very light negative record Will result, and thisnegative record, when printed, will produce a dense positive record. Thecurrents due to irregularities in the optical structure of the positiverecord, which are produced during the reproduction of the record, willthus be reduced. A reduction in these noise currents will permitmodulated currents of less than the present minimum amplitude toberecorded, thus the range of amplitude which may be recorded is furtherincreased.

A further feature of the invention resides in a novel form of lightvalve, in which two stretched ribbons are alternately displaced to formseparate records of the half-waves of the variations in amplitude of themodulated currents.

A developed photographic impression may be measured by permitting aknown intensity of light to fall on the impression and measuring theintensity of the light transmitted through the impression. The ratio ofthe original intensity of the light divided by the intensity transmittedthrough the impression is known as the opacity. The logarithm of theopacity is known as the density. The exposure of a photographic medium,or the light energy affecting the medium, is approximately equal to theintensity of the light impressed on the medium multipled by the timeduring which the medium is exposed tothat intensity of light.

Let a photographic medium be exposed to a graduated series of exposures,then developed and the densities produced by the various values ofexposures measured. The densities may then be plotted against thelogarithms of the relative exposures to produce the well known Hurterand Driflield curve of the medium.

If the H and D curve of a photographic medium is examined, it will befound that the density of the impression produced is not linearlyproportional to the logarithm of the exposure when the exposure issmall. This range of the characteristic is known as the region ofunder-exposure. At a fairly definite point, the curve becomes straightand the density of the impression produced is linearly proportional tologarithm of the exposure. This range of the characteristic is known asthe region of correct exposure. Above a certain value of exposure, thecurve again bends and the densities are no longer linearly proportionalto the exposures. This range of the characteristic is known as theregion of over-exposure.

In the known method of sound recording by the so-called variable densitymethod, in the absence of modulated currents the exposure of the mediumis adjusted to produce a density in the center of the region of correctexposure. The modulated currents then actuate the recording device andcause the exposure to Vary above and below this value. The maximumamplitude of the variations of the modulated currents which can berecorded without distortion, will be that amplitude which causes theexposure to vary over the whole region of correct exposure. That is, onehalf wave of the variation will cause the exposure to increase from thecenter point up to the point where the region of over-exposure commencesand then to decrease back to the center point. The other half wave ofthe variation will then cause the exposure to decrease from the centerpoint down to the point where the region of under-exposure commences andthen to increase back to the center point. Thus, each half wave maycause the exposure to vary over one half of the region of correctexposure.

In the present invention, separate records are employed for the two halfwaves of the variations. In the absence of modulated currents, theexposure of the medium of both records is adjusted to produce a densityat the lower end of the region of correct exposure. When a half wave ofthe modulated currents actuates the recording device, the exposure ofthe first record is virtually unchanged While. the exposure of thesecond record is increased. The maximum amplitude of the variations ofthe modulated currents which can be recorded without distortion willcause the exposure of the second record to vary over the whole region ofcorrect exposure. That is, the half wave causes a variation over thewhole region. Similarly, when the other half wave of the modulatedcurrents actuates the recording device, the exposure of the secondrecord is virtually unchanged and produces a density at the lower end ofthe region of correct exposure, While the exposure of the first recordis increased. Thus, each half-wave may cause the exposure to vary overthe whole region of correct exposure. That is, the amplitude of thevariations of the modulated currents which may be recorded withoutdistortion is approximately double the amplitude which may be recordedby present methods.

Recent experiments have shown that the noise currents due toirregularties in the photograph record produced during reproduction areapproximately proportional to the average transmission of the positiverecord. If the photographic processes are controlled so that no overalldistortion is produced, the transmission of the positive record isproportional to the exposure of the negative record. As hereinabovedescribed, in known methods of sound recording, the mean value of theexposure is adjusted to be in the center of the region of correctexposure. The noise currents produced in reproduction are thuscontparatively large. In the present invention, the exposure forcurrents of small amplitude is at the lower end of the region of correctexposure and is thus comparatively small. The noise currents arecorrespondingly small. Thus, modulated currents of smaller amplitude maybe recorded and still be distinguishable above the noise currents. Therange of amplitudes which may be recorded is thus extended due to thereduction in the absolute magnitude of the noise currents.

In a preferred embodiment of the invention, a light valve, comprisingtwo flat parallel ribbons immersed in a constant magnetic field, isemployed. The ribbons are stretched between insulating supports disposednear the outer ends of the ribbons. A central conductive bridge at thecenter of the ribbons forms an electrical connection between theribbons. Separate apertures in front of a moving photographic film arealigned with the centers of the two portions of the ribbons and areilluminated by a suitable source of light. The ribbons are mechanicallyadjusted so that they normally close the apertures to such an extentthat the resultant exposure will be at the lower end of the region ofcorrect exposure. Modulated currents are supplied to one end of oneribbon, fiow through half of that ribbon, through the conductive bridge,through half of the other ribbon and out of the end of the second ribbondistant from the point of supply. The reaction of the magnetic fieldformed by the modulated currents and the steady magnetic field cause thehalf ribbons to vibrate and alternately increase the light passingthrough the apertures.

In the drawing:

Fig. 1 diagrammatically shows a sound recording system embodying theinvention;

Fig. 2 is a partial sectional View of one form of light valve;

Fig. 3 is an enlarged view of the pole pieces,

The details or" the mechanical assembly and arrangement of the lightvalves shown in Figs. 2, 3, 4 and 5 may be similar to the light valvedisclosed in U. S. Patent 1,638,555 issued August 9, 1927 to E. C.Wente. Thus, it is not necessary to complicate the present drawing byshowing the minor mechanical details of the suspension and tensioning ofthe light valve-ribbons.

Acoustic waves are detected by a microphone I and produce a modulatedcurrent which is amplified in the amplifier 2. The modulated current maybe further amplified, if desired, in the amplifiers 3 and A. Theamplified currents are applied through wires 5, I to the ribbons E, 8of. the light valve.

Light from a suitable source 9 is focussed by a lens system II) onapertures I2, I3 pierced in the pole pieces of a permanent magnet II.The light passing through the apertures I 2, I3 is partially cut oil bythe ribbons Ii, 8 of the light valve. The light emerging from theapertures I2, I3 is focussed by a lens system id to form two images I5,IS on a photographic film I! moved at constant speed through the lightbeam in the usual manner.

The ribbons 6, 8 of the light valve. shown in Figs. 2 and 3 are securedat their center to a conductive bridge I8. As clearly shown in Fig. 2,the ribbons 6 and 8 are in two planes displaced from each other so thatif the ribbons are displaced past the center line they will not meet butwill overlap, ribbon 8 oscillating behind ribbon 6.

In Fig. 3 the lower edge of ribbon 6 is preferably aligned with theupper edge of the aperture I3 while the upper edge of ribbon 8 isaligned with the lower edge of aperture I2. In the position of rest, theribbons 6 and 8 are thus preferably spaced apart by a small distance.This distance may be arranged so that the light impressed on thesensitive film, when the light valve is in a position of rest, is justsufiicient to produce an exposure at the lower end of the region ofcorrect exposure of the emulsion used.

The modulated currents from Wire 5 flow along ribbon 6 to the bridge I8then along ribbon 8 to the wire I back to the amplifier 4. The righthand portion of ribbon 6 and the left hand portion of ribbon 8 do notcarry any current. Due to the reaction between the magnetic field formedby the modulated currents flowing in the ribbons and the steady magneticfield due to the magnet I I, the ribbons will be displaced. Assume thata half wave of the modulated currents causes the left hand portion ofribbon 6 and the right hand portion of ribbon 8 to move upward. Theremaining portions of the ribbons do not carry any current and will notbe displaced. Ribbon 6 will uncover a greater part of the aperture I2and the exposure of the record I5 will increase. Ribbon 8 will passunder ribbon 5 and the exposure of record I6 will decrease slightly andthen remain unexposed. The half wave of modulated current then decreasesto zero and the ribbons are restored to the position of rest. Ribbon 6has thus produced on record I5, a record of the first half wave of themodulated current while record I6 has at all times been virtuallyunexposed. The next half wave of the modulated current causes ribbons 6and 8 to be displaced downward. Ribbon 8'will uncover an increasing partof aperture I3 and the exposure of record IE will increase while ribbon6 will pass in front of ribbon 8 causing the exposure of record I5 todecrease slightly and then remain unexposed. When the second half Waveof the modulated current decreases to zero, both ribbons are restored tothe position of rest. Thus, the full range of the characteristic of theemulsion used may be used to record a half wave of the modulated currentand the volume of the recorded sound may be doubled. At the same time,the exposure applied to the negative film is at all times a minimum,thus the undesired noises produce-d during reproduction of a positiveprint of the negative record will also be a. minimum.

In Fig. 5, current from the amplifier 4 flows through wire 5, toconductive connector 2|, through ribbons I9 and 29 in parallel toconductive connector 22, wire I back to the amplifier 4. The wires I9and 29 may be spaced so that in their position of rest the exposureproduced on the negative film is at the lower end of the region ofcorrect exposure. When the first half wave of modulated current isapplied, assume both ribbons move upward. Ribbon 26 will uncoveraperture I 3 and the exposure of record I 6 will bevaried while apertureI2 is covered and record I5 is unexposed. On the second half wave ofmodulated current, both ribbons will move downward from the position ofrest, aperture 52 will be uncovered, the exposure of record !5 will bevaried, while aperture I3 is covered and record I6 is unexposed. In thetype of valve shown in Figs. 4 and 5 the overlapping of ribbons I9 and29 is not essential, but renders the adjustment of the ribbons easier.The apertures I2 and I 3 are also displaced slightly vertically from theposition of the apertures l2 and I3 in Fig. 3.

What is claimed is:

l. A light valve comprising a magnetic structure having parallel polefaces each pierced by a plurality of aligned apertures, a pair ofelectrical conductors supported between said parallel pole faces anddefining a light transmitting slot in each of said apertures, anelectrically conductive connectionbetween said conductors intermediatetheir ends, and means for supplying current to one end of one of saidconductors and to the other end of the other of said conductors to varytheir space relation whereby the light transmitted through said slots isalternately varied in accordance with the variations in the alternatehalf-waves of the current.

2. A light valve comprising parallel magnet having aligned pole faceseach pierced by a pair of aligned apertures, an electrical conductorsupported between said parallel pole faces and defining a lighttransmitting slot in each of said apertures, and means for supplyingmodulated current to said conductor to displace said conductor wherebythe light transmitting slots in said apertures are alternately varied inaccordance with the variations in the alternate half-waves of thecurrent.

3. A recording device comprising a photographic film moved at constantspeed, means for projecting light on said film, a light valve interposedbetween said means and said film, said valve comprising a magneticstructure pierced u by a pair of apertures defining said light into twoseparate beams and a member supported by said structure to define thebeam of light transmitted through each of said apertures, andelectrically operated means to vary the space relation of said memberwhereby the beams of light transmitted through said apertures arealternately varied in accordance with the variations in the alternatehalf waves of the electric current.

4. A light valve comprising a magnetic structure having parallel polefaces each pierced by two apertures, an electrical conductor supportedbetween said pole faces with one edge aligned with the upper edge of oneof said apertures, and substantially closing the second of saidapertures, a second electrical conductor supported between said polefaces with one edge aligned with the lower edge of the other of saidapertures and substantially closing the first of said apertures, anelectrical connection between said conductors, and means for supplyingcurrent to one end of the first conductor and the other end of thesecond conductor to cause said conductors to alternately open saidapertures in accordance with the alternate half waves of the current.

5. A light valve comprising a magnetic structure having parallel polefaces each pierced by two apertures, an electrical conductor supportedbetween said pole faces with one edge aligned with the upper edge of oneof said apertures and substantially closing the second of saidapertures, a second electrical conductor supported between said polefaces with one edge aligned with the lower edge of the other of saidapertures and substantially closing the first of said apertures, anelectrical connection between said conductors intermediate theirextremities, and means for supplying current to one end of the firstconductor and the other end of the second conductor to cause saidconductors to alternately open said apertures in accordance with thealternate half waves of the current.

6. A light valve comprising a magnetic structure having parallel polefaces each pierced by two apertures, the lower edge of the firstaperture being approximately aligned with the upper edge of the secondaperture, an electrical conductor supported between said pole faces withone edge aligned with the upper edge of the second aperture, a secondelectrical conductor supported between said pole faces with one edgealigned with the lower edge of the first aperture, a conductive bridgeretaining said conductors in different planes, and a circuit forsupplying current to one end of the first conductor and the other end ofthe second conductor to cause said conductors to alternately open saidapertures in accordance with the alternate half waves of the current.

'7. In a recording system, a source of light, a photographic film movedat constant speed, means for projecting light from said source on saidfilm including a magnet having parallel pole faces each pierced byapertures defining two beams of light, an electrical conductor supportedby said magnet in one plane, and obturating one of said beams, a secondelectrical conductor supported by said magnet in another plane andobturating the other of said beams, said con-- ductors beingelectrically connected, a source of signal currents, and a circuit forconveying signal currents to one end of said first conductor and. theother end of the second conductor to cause said conductors toalternately expose said beams in accordance with the alternate halfwaves of the signal currents.

8. In a recording system, a source of light, a photographic film movedat constant speed, means for projecting light from said source on saidfilm including a magnet having parallel pole faces each pierced byapertures defining two beams of light, each beam having sufiicientintensity to cause complete exposure of said film, an electricalconductor supported by said magnet and normally obturating one of saidbeams to produce an exposure of said film at the lower end of the regionof correct exposure, a second electrical conductor supported by saidmagnet and normally obturating the other of said beams to produce anexposure of said film at the lower end of the region of correctexposure, and means for applying signal currents to said conductorswhereby said beams are alternately varied in accordance with thealternate half waves of said currents.

9. The method of recording the variations in an electrical current whichcomprises projecting two beams of light uniformly over the full width oftwo different portions of a uniformly moving light sensitive medium,adjusting the minimum exposures of both said portions to a predeterminedsmall value, increasing the exposure uniformly over the full width ofone portion of said medium in accordance with an increase in the currentflowing in one direction, alternately increasing the exposure uniformlyover the full width of the other portion of said medium in accordancewith an increase in the current flowing in the opposite direction anddeveloping said medium.

10. The method of recording the variations in an electrical currentwhich comprises projecting .two beams of light uniformly over the fullwidth of two different portions of a uniformly moving light sensitivemedium, adjusting the minimum exposures of both said portions to be atthe lower end of the region of correct exposure of the medium, adjustingthe maximum exposures of both said portions to be within the region ofcorrect exposure of the medium, increasing the exposure uniformly overthe full width of one portion of said medium in accordance with anincrease in the current flowing in one direction, alternately increasingthe exposure uniformly over the full width of the other portion of saidmedium in accordance with an increase in the current flowing in theopposite direction and developing said medium.

IRL M. CHAMBERS.

